The invention pertains to methods of forming patterns across photoresist, and in particular applications pertains to methods of forming stencils and radiation-patterning tools; and to methods of patterning semiconductor substrates.
Photolithography is commonly used during formation of integrated circuits on semiconductor wafers. More specifically, a form of radiant energy (such as, for example, ultraviolet light) is passed through a radiation-patterning tool and onto a photoresist associated with a semiconductor wafer. The radiation-patterning tool can be, for example, a photomask or a reticle, with the term xe2x80x9cphotomaskxe2x80x9d being sometimes understood to refer to masks which define a pattern for an entirety of a wafer, and the term xe2x80x9creticlexe2x80x9d being sometimes understood to refer to a patterning tool which defines a pattern for only a portion of a wafer. However, the terms xe2x80x9cphotomaskxe2x80x9d (or more generally xe2x80x9cmaskxe2x80x9d) and xe2x80x9creticlexe2x80x9d are frequently used interchangeably in modern parlance, so that either term can refer to a radiation-patterning tool that encompasses either a portion or an entirety of a wafer. For purposes of interpreting this disclosure and the claims that follow, the terms xe2x80x9cphotomaskxe2x80x9d and xe2x80x9creticlexe2x80x9d will be given their historical distinction such that the term xe2x80x9cphotomaskxe2x80x9d will refer to a patterning tool that defines a pattern for an entirety of a wafer, and the term xe2x80x9creticlexe2x80x9d will refer to a patterning tool that defines a pattern for only a portion of a wafer.
Radiation-patterning tools contain light-restrictive regions (for example, totally opaque or attenuated/half-toned regions) and light-transmissive regions (for example, totally transparent regions) formed in a desired pattern. A grating pattern, for example, can be used to define parallel-spaced conductive lines on a semiconductor wafer. The wafer is provided with a layer of photosensitive resist material commonly referred to as photoresist. Radiation passes through the radiation-patterning tool onto the layer of photoresist and transfers the mask pattern to the photoresist. The photoresist is then developed to remove either the exposed portions of photoresist for a positive photoresist or the unexposed portions of the photoresist for a negative photoresist. The remaining patterned photoresist can then be used as a mask on the wafer during a subsequent semiconductor fabrication step, such as, for example, ion implantation or etching relative to materials on the wafer proximate the photoresist.
A method of forming a radiation-patterning tool is to provide a layer of light-restrictive material (such as, for example, chrome) over a light-transmissive substrate (such as, for example, a fused silica such as quartz), and subsequently etch a pattern into the light-restrictive material. The pattern can be etched by, for example, providing a photoresist masking material over the light-restrictive material, forming a pattern in the photoresist masking material with an electron beam or a laser beam, and transferring the pattern to the underlying light-restrictive material with an etchant that removes exposed portions of the light-restrictive material.
A typical photoresist material utilized for forming a radiation-patterning tool is chemically amplified photoresist. Such resist is particularly suitable for deep-ultraviolet (deep-UV) lithography, in that the resist can have high sensitivity. High sensitivity can be important as the light intensity of deep-UV exposure tools is typically lower than that of conventional i-line steppers.
In chemically amplified photoresist systems, a single photo-event initiates a cascade of subsequent chemical reactions. Typically, the photoresists are composed of an acid generator that produces acid upon exposure to radiation, and acid-labile compounds or polymers that have changed solubility in a developer solvent through acid-catalyzed reactions. The photoresist can be either a positive resist or a negative resist. A positive resist contains material which is initially relatively insoluble in a developer solvent, and which becomes soluble upon exposure to light-released acid; and a negative resist contains material which is initially relatively soluble in developer solvent, and which becomes insoluble upon exposure to light-released acid.
A difficulty that can occur during pattern formation with e-beam or laser writing on photoresist is that it can take several hours, and sometimes more than a day, for a pattern to be formed across an entirety of a photoresist expanse. Accordingly, a portion of a chemical-amplification photoresist expanse which is initially exposed to radiation will produce acid long before a portion of the photoresist expanse exposed to radiation at the end of the patterning process. The acid which is produced can diffuse into the photoresist expanse and cause minimum feature dimensions (i.e., critical dimensions) of initially exposed portions of the resist to be significantly larger than the minimum feature dimensions of later exposed portions of the resist. It would be desirable to develop methodologies which can alleviate or prevent such problems.
The problems described above with reference to radiation-patterning tool formation can also occur in other applications in which photoresist is sequentially exposed to radiation. For instance, in applications in which a reticle is utilized to transfer a pattern to photoresist, the reticle will be stepped across the photoresist to ultimately form the entire pattern on the photoresist. The portions of the photoresist initially exposed to light passing through the reticle can have chemical diffusion occurring therein for a longer period than the portions which are later exposed to light. Accordingly, critical dimensions associated with portions of the photoresist initially exposed to the light can be larger than the critical dimensions associated with portions of the photoresist which are later exposed to the radiation.
Additionally, methodologies have been developed wherein e-beam or laser writing technologies are utilized to form a pattern in photoresist directly over a semiconductor substrate. Such applications are similar to the above-discussed process of forming a radiation-patterning tool, except that they are utilized relative to a substrate comprising semiconductive material, rather than relative to a stack of quartz and chrome. The methodologies can suffer from the problems described above with reference to e-beam and laser writing applications for radiation-patterning tool formation.
In one aspect, the invention encompasses a method for forming a pattern across an expanse of photoresist. The expanse comprises a defined first region, second region and third region. The first region is exposed to a first radiation while leaving the third region not exposed; and subsequently the second region is exposed to a second radiation while leaving the third region not exposed to the second radiation. The second radiation is different from the first radiation. The exposure of the first and second regions of the expanse to the first and second radiations alters the solubility of the first and second regions in a solvent relative to the solubility of the third region of the expanse. After the first and second regions of the expanse are exposed to the first and second radiations, the expanse is exposed to a solvent to pattern the expanse.
In another aspect, the invention encompasses a method of forming a radiation-patterning tool. A radiation-patterning tool substrate is provided, and comprises an opaque material over a quartz plate. A layer of chemical amplification photoresist is formed over the radiation-patterning tool substrate. A first region, second region and third region are defined within the layer of photoresist. The first region is exposed to a first dose of radiation while leaving the third region not exposed to the first dose; subsequently the second region is exposed to a second dose of radiation while leaving the third region not exposed to the second dose. The second dose is different from the first dose. The exposure of the first and second regions of the photoresist to the radiation alters the solubility of the first and second regions in a solvent relative to the solubility of the third region. After the first and second regions of the layer of photoresist are exposed to the radiation, the photoresist is exposed to the solvent to pattern the layer of photoresist. Subsequently, a pattern is transferred from the photoresist to the opaque material to form a radiation-patterning tool from the radiation-patterning tool substrate.